chore(CONTRIBUTING): add documentation to contributing
chore(CONTRIBUTING): add documentation to contributing

references #6172

joshesp/ionic,jarrodkoch/ionic,alan-agius4/ionic,janglednerves/ionic,TillaTheHun0/ionic,jhlee8804/ionic,AbraaoAlves/ionic,joshesp/ionic,janglednerves/ionic,cambronjay/ionic,alan-agius4/ionic,jhlee8804/ionic,jarrodkoch/ionic,AbraaoAlves/ionic,cambronjay/ionic,alan-agius4/ionic,AbraaoAlves/ionic,AbraaoAlves/ionic,jhlee8804/ionic,janglednerves/ionic,cambronjay/ionic,dennybiasiolli/ionic,cambronjay/ionic,manucorporat/ionic,jhlee8804/ionic,manucorporat/ionic,alan-agius4/ionic,joshesp/ionic,manucorporat/ionic,dennybiasiolli/ionic,dennybiasiolli/ionic,brianespinosa/ionic2-sass,cambronjay/ionic,janglednerves/ionic,driftyco/ionic,jarrodkoch/ionic,jhlee8804/ionic,TillaTheHun0/ionic,jarrodkoch/ionic,dennybiasiolli/ionic,AbraaoAlves/ionic,dennybiasiolli/ionic,janglednerves/ionic,manucorporat/ionic,TillaTheHun0/ionic,driftyco/ionic,alan-agius4/ionic,jarrodkoch/ionic,driftyco/ionic,TillaTheHun0/ionic,alan-agius4/ionic,manucorporat/ionic,driftyco/ionic,joshesp/ionic,driftyco/ionic